1. Field of the Invention
The invention herein consist of a control mechanism for the exit area of convergent-divergent nozzles. The mechanism allows the control of the nozzles exit area in convergent-divergent axisymmetric nozzles, independently of the throat area and is specially design for gas turbine engines used as aircraft propulsion systems.
The invention consists of a nozzle structure (2), on which a synchronising ring (1) is allowed to rotate and to move in the axial direction. Angled lever arms (3) with pivoting capability (4) on the nozzle structure (2), are connected on one side to the synchronising ring (1) and on the other to compression struts (8), joined to the divergent petals (10). The rotation of the synchronising ring (1) around the nozzle structure (2) is converted by the angled lever arms (3) in a predominant axial displacement of one side of the compression struts (8), resulting in the opening or closure of the divergent petals (10).
This mechanism, because of being able to control the exit area of the nozzle can achieve an increased number of optimised point in the aeroplane flying envelope.
2. Description of the Related Art
A gas engine propulsion system produces an axial thrust by momentum change of the high speed exiting gases though the aeroplane exit nozzle. The air enters the engine though the compressor where it is compressed. It is then heated by fuel combustion. The hot exiting gas is expanded in the turbine, obtaining work which is used to drive the mentioned compressor. The gas expansion continues in the nozzle, where the remaining energy on the gas is converted into a high velocity stream, responsible for the engine thrust.
The nozzles currently employed in aeroplanes can be divided in the following groups, attending to their complexity. In civil aviation, it is usual to have convergent nozzles with a fix ratio between entering and throat areas. In military engines with reheat capability, it is necessary to have a system that can allow the change in throat area. Some engines have a divergent section after the convergent, which can continue the gas expansion above sonic velocities, achieving an increased thrust and a reduction in specific fuel consumption. Most of the convergent- divergent nozzles have a single degree of freedom, so that for each throat area the exit area is determined based on the length of the petals and compression struts. The mentioned dimensions and the relation between throat and exit areas is determined as the best fit curve of ideal ratios, which are a function of cruise height and mach number for a stationary flight. Choosing a single area law means that for some flying conditions the thrust or specific fuel consumption can be penalised by as much as 5% of total engine thrust.
The present invention describes a mechanism capable of changing the exit area independently of the throat area, allowing the optimisation of thrust in all flying conditions, without a significant weight increment or an unreliable failure mode.
Both convergent and divergent sections are made up of independent petals that once installed work simultaneously.
The convergent petals are mounted individually through a cylindrical joint on the exit perimeter of the engine. The divergent petals are joined by a cylindrical joint to the rear end of the convergent petals. Each of the divergent petals is joined to a compression strut. Usually this compression strut is joined on its other end to the nozzle structure. All four joints are parallel to each other and perpendicular to the nozzle axis, such that each set of petals and strut make up a four link mechanism. The actuation of the mechanism generally consists of the rotation of the convergent petal around its joint with the engine structure. This four link mechanism allows the variation of the throat and exit areas simultaneously, with a single degree of freedom, upon a fixed ratio determined by the length of the elements that make it up.
The present invention introduces a mechanism between the strut and the engine structure that can vary the axial distance between them gaining an additional degree of freedom in the four link mechanism petal arrangement allowing the control of the exit area independently of throat area. The mechanism therefore allows the optimum expansion of the exit gases and optimum thrust, in other words an increased number of optimised flying conditions.
According to the invention, the exit area control mechanism for convergent- divergent axisymmetric nozzles, consist of a synchronising ring, concentric with the nozzle axis, which is joint to a group of lever arms that pivot around a preferably radial axis around the nozzle structure, determining the axial position of a group of struts that are joined at one end to the end of the mentioned lever arms and on the opposite end to a group of divergent petals, such that for every circumferential position of the synchronising ring the exit area of the axisymmetric nozzle is determined.